Extractive separation process



United States Patent EXTRACTIVE SEFARATION PRQQESS David Cornell, Stillwater, Okla, assignor to Monsanto Company, a corporation of Delaware No Drawing. Filed lune 26, 195i, Ser. No. 1l,268 15 Claims. (Cl. 292-495) The present invention relates generally to the separation, concentration, and/ or purification of hydrocarbons having Various degrees of unsaturation. It is an object of the invention to separate close-boiling hydrocarbons of the classes of paraflins, monoolefins, diolefins, naph-' thenes and aromatic hydrocarbons by-the use of. phos phorus amides having the following structural formula:

preferably having from i to 4 carbon atoms, and R" is selected from the class consisting of alkyl and cycloalkyl radicals. it is also an object of the invention to carry out the said separations by means of an extractive distillation method employing the said phosphorus amides as the extracting agent.

In -a number of hydrocarbon processing operations including cracking, genating, a wide spectrum of hydrocarbons is formed having various degrees of unsaturation or of solubilityparameter, cohesive energy density, or internal pressure. It is therefore desirable to be able to make a type separation in order to remove substantially all of each individual family group of hydrocarbons, i.e., the parafiins, monoolefins, cliolefins, naphthenes and aromatic hydrocarbons. Further separations such as one olefin from another is also a desired objective. Conventional distillation methods are .often poorly adapted to the separation and recovery of such classes of hydrocarbons in view of the'small differences in the boiling pointsofthe respective compounds. it has also been found that azeotropic distillation in which the weotrope agents are added to reduce the boiling point of certain components is impractical because of the separation dfiiculties between such agents and the compounds with which the azeotrope has been formed.

It has now been found that the use of the abovedescribed phosphorus amides makes its possible to conduct extractive separations among the above-described classes of hydrocarbons. In carrying out the presentinven tion utilizing the said phosphorus amides an extractive distillation process the phosphorus amide compound or a mixture thereof is introduced into a distilla' I tioncolurnn at a point near the top of the column. In

this case theone hydrocarbon fraction is withdrawn as the overhead product, while the other hydrocarbon group is obtained as a bottom product dissolved in the phosphorus amides as the extractive distillation solvent;

The present method is applicable to the separation of hydrocarbons of'the classes of parafiins;monoolefins, diolefins, naphthenes and aromatic hydrocarbons, as well as many individual members within sucha class, for example, -butene-Z from butene-l. The extractive distillation process using the phosphorus amidesyields a vapor fraction containing the more volatile of the said hydrocarbons. The volatility here referred to is that of the hydrocarbon reforming, aromatizing and :dehydro when in'so-lution in the phosphorusarm'de', such volatility .being the productq P Where the the activity coefli= cient and R is the vapor pressure of the hydrocarbon.

Relative volatility,-- a, is, therefore the ratio of the .Pi, products for two hydrocarbons. T t

as an extractive process with a wide variety of crude It has been found thatthe present'method is efiicacious hydrocarbon mixtures.

number of p-lates and other details of sufficiently dilierent,

hydrocarbon (s) bottoms.

Examples of such starting mixtures include the paraffin, monoolefin, and diolefin mixture obtained in the dehydrogenation of butane and butene in order to produce butadiene as the desired product. Another type of crude hydrocarbon mixture which is readily separated by the method of this invention is the octane-octene mixture resulting from the dehydrogenation of a C fraction. Another type of hydrocarbon fraction which can be separated by the present process is a mixture of 5 carbon atom hydrocarbons obtained in a dehydrogenation of pentenes including isopentane in order to produce isoprene. The impurities in such a crude mixture include normal pentane, pentene-l, Z-methylbutene-l, and Z-methylbutene-Z. Another crude hydrocarbon mixture readily separated by means of the present method is the mixture of naphthenes and'aromatic hydrocarbons obtained in the aromatizing of normal hexane andv the subsequent dehydrogenation of such crude mixture to produce benzene.

The proportion of the mixture of phosphorus amides employed in the present extractive separation method varies over the range of from 0.5 to 10 moles of the said phosphorus amide per mole of the crude hydrocarbon mixture, a preferred range being from 1 to 5 moles. The separation processes may be operated over a wide range of temperatures such as from F. to 300 F., the upper temperature being limited by the tendency of the hydrocarbon to polymerize rather than by any inherent limltationof the extractive distillation process. The use of vacuum or pressure conditions, in addition to atmosphonic pressure is process is typical of the equipment available in this held. I It is obvious that such a distillation process may be conducted with any conventional distillation column of the bubble-plate, packed, or sieve-plate type as may be desired. The selection of the best reflux ratio, size and column design necessary in order to obtain the desired degree or" purity will be obvious to one skilled in the art having the benefit of the present disclosure. If necessary to prevent or the polymerization of unsaturated compounds, conventional polymerization inhibitors may also be used.

The apparatus employed constitutes a conventional extractive distillation column 'inwhich the crude mixture r" hydrocarbons ischarged to the middle region of a column with reflux column, .while the overhead vapor fraction is withdrawn as an enriched stream of the material with the higher degree of saturation (or lower solubility parameter). The phosphorus amide solvent from any source is introduced into the column ataplate located several plates below the top of the column.

The bottoms stream leaving the column contains the material with the lower degree-of saturation or higher solubility the phosphorus amide solvent.

Where more than one class of hydrocarbons is "present in the vapor, and/or liquid fractions, these fractions may be separately further treated with the extractive distillation solvent ,-to effect further hydrocarbon separations or," wherethe boiling points or miscibllitie'srof the varioushydrocarbons are In subsequent extractive distillations the more volatile is withdrawn as overhead .vapors and theless' volatile hydrocarboms) is withdrawn as liquid The mixture of solute and solvent in the bottoms fraction is then separated intoits components by being returned near the top of the parameter, together with other techniques .such jas fractional distillation or solvent extraction separation are suitable;

conventional stripping or separation means, which may comprise the use of water washing, solvent extraction, distillation, or freezing, by which means one may obtain the bottoms solute in the desired pure state. For example, one may employ a conventional fractionation or stripper column, wherein by simple fractional distillation the solute from the bottoms product is recovered as the overhead fraction of the stripper in pure form. In another type of column the bottoms solute in admixture with the solvent is fed into the middle region of a column, while steam or another heated inert gas is fed to the bottom of the column. The overhead product from such stripping operation is the pure solute, while the solvent is obtained as the bottoms product which is then dried and recycled to the main distillation column, as described above.

It has been found that the instant phosphorus amides are particularly advantageous in the present process, since these materials are relatively stable against decomposition and are non-reactive with respect to the hydrocarbons as Well as any impurities which are conventionally found in such crude mixtures. It is also an advantage that the phosphorus amides are generally relatively non-toxic and are relatively inexpensive materials. The use of phosphorus amides as herein disclosed makes it possible to separate close-boiling hydrocarbons in a considerably smaller column than would be required for conventional distillation.

The comparative selectivity of an extractive distillation solvent is best determined by its specific efficiency with respect to the hydrocarbon pair which are to be separated in the present method. This efiiciency may be expressed as the relative volatility of the two hydrocarbons in the presence of the phosphorus amides solvent. The

equation which expresses this relative volatility (alpha) is:

Jim- 1w a (7 v) hydrocarbon 2 where (gamma) represents the activity coeflicients defined by the following equation:

' YIPT 7 X.Pv.

In the above equation X and Y represent the mole fractions of a given component in the liquid and vapor phase, respectively, while P and P represent the vapor pressure of the given component, and the total pressure of the system respectively.

In addition to selectivity, solubility of hydrocarbons in the phosphorus amides must be considered. The. quantity =(AE /V) E=internal energy of vaporization, calories/ (g. mole) V =molal liquid volume, cc./ (g. mole) I V Forthe condition of ideal gases, AE may be calculated.

from handbook values of the latent heat of vaporization, 23,. The temperatures are expressed as degrees, Kelvin.

. AE=AH,,RT AH,-= latent heat of vaporization, calories/ (g. mole) f R=l.-987 calories/(g. mole) K.)

T absolute temperature, K.

It has been found by means of solubility measurements, standardized at77 F., the naphthenes or aromatic hydrocarbons with a solubility parameter greater than about 8.4 are quite soluble in the above-described phosphorus amides. Parafiin or naphthene hydrocarbons with lesser solubility parameters are much less miscible. It has also been found that the solubility parameters given above are affected by the temperature of the system. Monoolefins and diolefins exhibit much greater miscibility with the of the invention:

phosphorus amide solvents than corresponding parafiin hydrocarbons. Accordingly, a separation can be effected through either a difference in degree or saturation or in the case of parafiin, naphthene and aromatic compounds, through a difference in solubility parameter. In some cases, e.g., a mixture of C hydrocarbons, natural volatility ratios of some constituent pairs are inverted by the presence of the solvent.

The following examples illustrate specific embodiments EXAMPLE 1 A number of hydrocarbon type mixtures are employed in order to demonstrate the selectivity of cyclohexyl N,N- dimethyl-P-methylphosphonamidate as an extractive distillation solvent. These tests are conducted at a number of temperatures as set forth in the table below. At the said temperatures at which the equilibrium measurements are made, at the solvent ratio set forth in the table, the relative volatility of the two components is determined. These values of relative volatility are defined inaccordance with the description above.

The table of data also shows the enhancement per plate obtained when using a 10% (mole) solution of the more volatile component of each pair with of the less volatile in an extractive distillation using cyclohexyl N,N- dimethyl-P-methylphosphonamidate as the solvent. 7

Enchancement per plate is calculated according to the expression aX Y- 1 (112- l X the percentage enhancement being lOOY. In the expression, Y and X refer to vapor and liquid molar compositions, respectively, of the more volatile hydrocarbon, taken on a solvent-free basis. Thus Y=0.129 as opposed to X =0.100 indicates an enhancement of 12.9%.

Table I En- Solv. Ratio: Rela hance- Run Solute Temp, Vol. S0l./ tive merit F. Vol. Vola- Per Solutes tility, a Plate percent n-Pentane- 1. 33 12. 9 its en ane- 2 PBDtBDG-l. 1. 00 n-Pentane 3. 75 29. 4 3 2-Methylbutene-1 150 2 2.14 19.2 Z-Methylbutene-Z 1.13 10. 1

n2 nctaue 4 g land i 8% in nexane 5 {Oyclohexane 169 1.00

Butane. 1. 88 17. 3 6- Butene- 3 l. 92 17. 6 Butadiene- 1. 00 Methyleyclopentane. 2. 79 23. 8 7 n-Hexane 15H 1 6. 65 42. 5 Benzene. 1. 00 2,4-Dimethylpentane 3. 95 30. 5 8 Cyclohexane" 150 0. 5 1.30 12.6 Benzene 1. 00

EXAMPLE 2 usinga 10% '(mole) solution of the more volatile coin-g ponent of each pair with 90% of the less volatile in an ex Table II Eu- Solv. Ratio: Relahance- Run Solute Temp, V01. SoL/ tive ment F. Vol. Vola- Per Solutes tility, a Plate percent n-Pentane. 107 10. 6 1 Pentene-L. 150 1. 00 n-Pentane 1. 85 17. 1 2 2-1/Iethylbutene-1 150 12. 6 1. 50 14. 3 Isoprene 8g i u-Hexane 3 .Cygflohexanm 190 82 i l1- QXElIl8 0 4 Methylcycloo tanc 199 38 1. 00 l3 1.45 13. 9 i 150 9 1.58 14. 9 Butadiene 1.00 Me thylcyclopentane. 2. 54 22. 0 6 u-I-Iexane 150 9 5.62 38. Benzene 1. 0O 2,4-Dimethylpentane 2. 20 19. 7 7 Cyelohexane 150 5 1. 45 13. 9 Benzene 1. 0O

EXAMPLE 3 The use of hexyl N,N-dimethyl-P-ethylphosphonamidate as the extractive distillation solvent in the separation of normal pentane from pentene-l is shown in the following test data. The table of data also shows the enhancement per plate obtained using a 10% (mole) solution of the more volatile component of each pair with 90% of the less volatile in an extractive distillation using hexyl N,N-dimethyl-P-ethylphosphonamidate as the solvent.

Table II! Enhance- Solvent Relative mcnt Solute Temp, Ratio: Vol. Volatil- Per F. Solvent/V o1. ity, 0: Plate solutes (Percent) n-Pentane 1. 10 10, 9 Pentene-l 150 1.00

A demonstration of the effectiveness of the phosphorus amide solvents described herein may be made by reference to data on the hydrocarbon selectivity of a 2/1 volume ratio acetone-water solvent as shown in the following table:

Table IV Selectivities and relative volatilities listed in the table.

are seen to be substantially less using the acetone-water solvent than the phosphorus amides of the instant invention.

The phosphorus amides usedas solvents herein may readily be prepared by conventional means known in the art. For example, phosphorus amidesare prepared by reacting amines or alcohols with phosphorus oxyhalides, halophosphates, halo amidophosphates and phosphonyl halides as more particularly detailed in Kosolapoif, Organophosphorus Compounds, Wiley & Sons, Inc. (1950),

chapter ten, sections II and X, and references cited therein.

The above data demonstrates that the hereinbeforedescribed phosphorus amides are highly effective asextractive distillation solvents in the separation ofhydrocarbon mixtures.

What is claimed is:

1. The method of separating classes of hydrocarbons from a mixture comprising hydrocarbons of the classes consisting of paraffins, monoolefins, diolefins, n aphthenes, and aromatic hydrocarbons which comprises contacting the said mixture with a phosphorus amide having the following structural formula:

wherein R and R represent alkyl radicals and R" is selected from the group consisting of alkyl and cycloalkyl radicals in an extractive distillation separation, withdrawing a vapor fraction con'aining the more volatile of the said classes, and also withdrawing a liquid fraclion containing the less volatile classes dissolved in the said phosphorus amideand thereafter separating the individual hydrocarbon classes from each of said fractions by conventional means and stripping the said phosphorus .amide from the hydrocarbons dissolved therein.

2. The method of separating piaraflins from olefins which comprises contacting a mixture thereof with a phosphorus amide having the following structural formula:

..afiin is pentane and the olefin is pen'tene.

4. The method according to claim 2 wherein the phosphorus amide is cyolohexyl N,N-dimethyl-P-methylphosphonamidate.

5. The method according to claim 2 wherein the phosphorus amide is methyl N,N-dibutyl-P-hexylphosphonamidate.

6. The method according to claim 2 wherein the phosphorus amide is hexyl N,N-dimethyl-P-ethylphosphon amidate.

7. The method accbrding to claim 2 wherein the pan affin is octane and the olefin is octene.

8. The method of separating diolefins from a mixt 6 comprising hydrocarbons of the classes consisting of paraflins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons which comprises contacting the said mixture with a phosphorus amide solvent having the following stnuotural formula:

0 Phi -0B" 1\IT(R')2 wherein R and Rv represent alkyl radicals and R" is selected from the group consisting of alkyl and cycloalkyl radicals in an extractive distillation separati'om' withdrawing a vapor fraction containing the more volatile paraffins, monoolefins, diolefins and naphthenes and also withdrawing ;a liquid fraction containing the less volatile aromatic hydrocarbons dissolved in the said phosphorus amide solvent and thereafter contacting the l1ydrocarbon mixture comprising said vapor fraction'jwith additional amounts of said phosphorus amide solvent in' a second extractive distillation to removeithe more volatile paraiiins, naphthenes and monoolefins as a vapor fraction from the less volatile diolefins dissolved in said solvent as a liquid fraction and thereafter stripping the said phosphorus amide from the diolefins dissolved there- III.

9. The method according to 8 wherein the diolefin is isoprene.

10. The method according to claim 8 wherein the diolefin is hutadiene.

11. The method according to claim 8 wherein the phosphorus amide is cyclohexyl N,N-dimethyl-P-methylphosphonamidate.

12. The method of separating paraflins from a mixture comprising hydrocaubons of the classes consisting of parafiins, monoolefins, diolefins, naphthenes and aro-' matic hydrocarbons which comprises contacting the said mixture with a phosphorus amide having the following structural formula:

E RI"0R" wherein R and R represent alkyl radicals and R" is selected from the group consisting of alkyl and cycloalkyl radicals in an extractive distillation separation,

v withdrawing a vapor fraction containing the more volatile par-affins and also withdrawing a liquid fraction containing the less volatile olefins, naphthenes and aromatic hydrocarbons dissolved in the said phosphorus amide and thereafter stripping the said phosphorus amide from the hydrocarbons dissolved therein.

13. The method according to claim 12 wherein the parafiin is hexane and the phosphorus amide is cyclohexyl N,N-dimethyl-P-methyilphosphonamidate.

14. The method of'separating aromatic hydrocarbons from a mixture comprising hydrocarbons of the classes consisting of paraffins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons which comprises contacting the said mixture with a phosphorus amide having the following structural formula: i

O wherein R and. R represent alkyl radicals and R is selected from the group consisting of alkyl and cycloalkyl radicals in an extractive distillation separation, withdrawing a vapor fraction containing the more volatile parafiins, olefins and naphthenes and also withdrawing a liquid fraction containing the less volatile aromatic hydrocarbons dissolved in the said phosphorus amide and thereafter stripping the said phosphorus amide from the aromatic hydrocarbons dissolved therein.

15. The method according to claim 14 wherein the aromatic hydrocarbon is benzene.

References Cited in the file of this patent UNITED STATES PATENTS Technique of Organic Chemistry, vol. IV, Distillation (Weissberger), published by Interscience Publishers, Inc. (New York), 1951, pp. 338439.

UNITED :STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,158 ,555 November 24, 1964 David Cornell It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1 lines 15 to 18, the formula should appear as shown below instead of as in the patent:

R-P-O-R column 3, line 68, for "A read AH --;-.aco'lumnu4,ll'ine if), for "or" read of column 5, Table II, under the heading "Relative Volatility, on", line 1 thereof, for "107" read l'.07

Signed and sealed this 24th day of August 1965.

(SEAL) Attest:

ERNEST W. SW IDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. THE METHOD OF SEPARATING CLASSES OF HYDROCARBONS FROM A MIXTURE COMPRISING HYDROCARBONS OF THE CLASSES CONSISTING OF PARAFFINS, MONOOLEFINS, DIOLEFINS, NAPHTHENES, AND AROMATIC HYDROCARBONS WHICH COMPRISES CONTACTING THE SAID MIXTURE WITH A PHOSPHORUS AMIDE HAVING THE FOLLOWING STRUCTURAL FORMULA: 